Charged particle generating and utilizing

ABSTRACT

A cylindrical, retarding potential difference type electron monochromator with exceptionally high energy resolution includes an electron beam source having essentially zero transverse velocity in a region that is maintained essentially free from magnetic fields. This device can also be used as an electron energy analyzer. In the electron spectrometer the electron beam passes through a chamber having a gas to be analyzed and then impinges upon an electron collector or electron energy analyzer. A sawtooth scanning potential waveform is applied to the chamber; a square wave of higher frequency, to a retarder electrode. The collector current as a function of the instantaneous value of the sawtooth potential measures the electron resonances of the gas. The electron resonances identify the various constituents of the gas and determine the partial pressures of the individual constituents. The device described herein to produce a monoenergetic electron beam or to energy analyze an electron beam applies equally well to other charged particles such as atoms or molecules.

[ 51 June 13, 1972 [54] CHARGED PARTICLE GENERATING AND UTILIZING [72] Inventor: David E. Golden, Cambridge, Mass.

[73] Assignee: Advanced Research Instrument Systems,

Inc., Austin, Tex.

[22] Filed: April 20, 1970 [2 1] Appl. No.: 30,075

[52] US. Cl. ..250/43.5 R, 250/495 R [5 l] Int. Cl. ..G0ln 23/02 [58] Field of Search ..250/49.5 R, 49.5 PE, 49.5 P, 250/435 R; 328/233 [56] References Cited OTHER PUBLICATIONS High Resolution Low Energy Electron Spectrometer," Simpson, The Review of Scientific Instruments, Vol. 35, No. 12, Dec. 1964, pp. 1,698- 1,704

Scanning Electron Diffraction With Energy Analysis," Denbigh et al., Journal of Scientific Instruments, Vol. 42, [965, pp. 305- 311 PRESSURE Primary Examiner.lames W. Lawrence Assistant ExaminerC. E. Church Attorney-Charles Hieken [57] ABSTRACT A cylindrical, retarding potential difierence type electron monochromator with exceptionally high energy resolution includes an electron beam source having essentially zero transverse velocity in a region that is maintained essentially free from magnetic fields. This device can also be used as an electron energy analyzer, In the electron spectrometer the electron beam passes through a chamber having a gas to be analyzed and then impinges upon an electron collector or electron energy analyzer. A sawtooth scanning potential waveform is applied to the chamber; a square wave of higher frequency, to a retarder electrode. The collector current as a function of the instantaneous value of the sawtooth potential measures the electron resonances of the gas. The electron resonances identify the various constituents of the gas and determine the partial pressures of the individual constituents. The device described herein to produce a monoenergetic electron beam or to energy analyze an electron beam applies equally well to other charged particles such as atoms or molecules.

HELMHOLZ COIL VACUUM SYSTEM E HELMHOLZ COIL 22 SQUARE WAVE PULSER 2| PHASE SENSITIVE M DETECTOR FIG. I

SMUIBFT VACUUM SYSTEM GAUGE PATENTEDJux 13 m2 PRESSURE GAS SAM PLE V0 (VH-LVf) FIG. 2B

INVENTOR DAVID E. GOLDEN ATTORNEY VrVo(Vr+ AV FIG. 2D

Vr lo Fl G. 2A

Vr Vo(Vr+ A Vr) FIG. 2C

P'ATEN'TEDJuH 13 m2 SHEET 2 OF 7 vm mm mm 2m m INVENTOR DAVID E. GOLDEN ATTORNEY P'A'TE'N'TEDJuu 13 m2 SHEET t UP 7 (mm) (meW TRANSMITTED CURRENT Z ERO SUPPRESSED ARBITRARY Y Y I I I I I I Vr =0.03 vols UNITS) L9 DC u to 0 1 0 LL. Lu J Lu INVENTOR DAVID E. GOLDEN ATTORNEY PAIEIIIEIIJUII 13 m2 3.670.172 SHEET 501- 7 GAS INLET VALVE PRESSURE GAUGE FARADAY CUP SHIELD/ ELECTRON Wm El CHROMATOR GAS CELL FARADAY CUP VAC UUM SYSTEM TO SCATTERING CELL TO RETARDER ELECTRODE OF MONOCHROMATOR COM MAND PHASE s|e NAL SENS'TWE To FARADAY CUP GENE RATOR DETECTOR X AXIS OF XY RECORDER X Y RECORDER OSCILLOS COPE FIG. 8

INVENTOR DAVID E. GOLDEN ATTORNEY FATENTEDJM 1 3 m2 3.670.172

SHEET 5 OF 7 GAS INLET VALVE PRESSURE K GAUGE ELECTRON -l ELECTRON MONO ENERGY CHROMATOR ANALYZER GAS CELL PUMP RAMP GENERATOR TO ENERGY ANALYZER SQUARE WAVE TO RETARDER ELECTRODE SQUARE WAVE TO RETARDER ELECTRODE OF ENERGY ANALYZER XY RECORDER OR SCOPE Fl G. 9

iNvENToR DAVID E. GOLDEN AT TOR NE Y PATENTEDJIIII 13 I972 SHEET 7 [IF 7 SAMPLE PROCESS SPECTROMETER A OR B COMMAND REF PHASE SIGNAL GNAL SENSITIVE GENERATOR S DETECTOR ANALOG AND DIGITAL CONVERTER SPECTRA LOGIC FILE COMPARATOR PRINT OUT COMPOSITION FIG. ll

PROCESS CONTROLLER INVENTOR DAVID E. GOLDEN ATTORNEY BACKGROUND OF THE INVENTION The present invention relates in general to charged particle generating and utilizing and more particularly concerns a novel electron monochromator and electron energy analyzer of the retarding potential difference type characterized by exceptionally high energy resolution. The invention is especially useful for measuring resonances in atoms or molecules at low electron energies, (from I through 60 electron volts). Furthermore, the same device can be used to produce a highly monoenergetic beam of any charged particles. Conversely the device can be used to energy analyze any beam of charged particles.

In low energy electron spectroscopy an electron beam of well-defined energy impinges upon a gas whose resonances are to be measured. At a resonant energy level the number of scattered electrons changes sharply with energy, since the energy positions of these resonances are different for different atomic or molecular species. The resonant positions can be used to identify the constituents of the gas. In addition the change in transmitted electron current at resonance yields the partial pressures of the gas constituents.

A more precise way of making these measurements is known as the retarding potential difference (RPD) technique. A retarding electrode receives a d-c retarding potential just sufficient to prevent electrons from reaching the gas cell containing the substance to be analyzed. By applying a square wave potential upon the retarder electrode, the retarder electrode alternately passes to and blocks the electrons from a gas cell. By differentially combining the collected current when the retarder electrode potential is just below cutoff with the collected current when the retarder potential is just above cuttoff, the resultant difference current contains only electrons whose energies vary only by an amount AE =eAV, where e is the charge of the electron and AVis the peak to peak value of the applied square wave on the retarder electrode.

It has been discovered that the achievable energy resolution of this (the RPD) method is related to the degree of parallelism in a beam of low energy charged articles. Since the device is to be used at very low energies and a beam of highly monoenergetic particles is desired, even extremely small transverse velocities are significant.

In a typical prior art approach an axial magnetic field aligned the charged particles. This magnetic focusing approach does not eliminate the transverse components of velocity, but causes the electrons to follow helical paths with the radius of each helix proportional to the product of the mass-to-charge ratio and the transverse-velocity-to-axial-magnetic-field-strength ratio. In fact it is the presence of the transverse component of velocity which has placed the upper limit on state of the art RPD monochromators. As a practical matter that approach has produced electron beams with full energy spectrum width at half maximum current of greater than 0.l eV, thereby limiting the resolution of any associated instrument such as an electron spectrometer or electron energy analyzer.

Accordingly, it is an important object of this invention to provide an exceptionally high energy resolution charged particle monochromator based on the elimination of the transverse velocity component in any charged particle beam.

It is a further object of the invention to provide a charged particle energy analyzer of extremely high energy resolution.

It is a further object of the invention to achieve one or more of the preceding objects with a charged particle beam of relatively low energy, although the invention can also be used to achieve one or more of the preceding objects with a charged particle beam at relatively high energy.

It is a more specific object of the invention to provide a beam of like charged particles that travel along paths parallel to each other.

It is a further object of the invention to achieve one or more of the preceding objects with a relatively high density chargedparticle beam.

It is still a further object of the invention to achieve one or more of the preceding objects in a region that is free from magnetic fields.

It is still a further object of the invention to provide an RPD spectrometer in accordance with one or more of the preceding objects characterized by exceptionally high resolution.

SUMMARY OF THE INVENTION According to the invention, there is a source of charged particles traveling along closely adjacent parallel paths forming a charged particle beam. Cell means for carrying a substance to be analyzed is positioned to intercept the charged particle beam. Collector means or other suitable energy analyzing means is arranged for receiving those charged particles that pass through the cell means. A measure of the charged particles which pass through the cell means as a function of its potential indicate the charged particle resonance characteristic of the substance in the cell means.

Preferably there is means, such as a Helmholz coil or mumetal shield or other type of magnetic shield, for establishing a region in which said particle beam passes that is essentially free from magnetic fields.

According to a more specific aspect of the invention there is a charged particle gun comprising a series of axially symmetric electrostatic lens elements having means defining an output aperture for eliminating axial velocity components of said charged particles as they pass through said exit aperture.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a combined block-diagrammatic representation of a charged particle spectrometer consisting of electron monochromator, gas cell, and electron collector according to the invention;

FIGS. 24, 2b, 2c and 2d are graphical representations of collector currents as a function of the potential on the retarder electrode of FIG. 1 helpful in understanding the principles of the RPD spectroscope;

FIG. 3 illustrates a charged particle monochromator according to the invention for producing a charged particle beam at its exit aperture free from transverse particle velocity components, a gas cell, and electron collector;

FIG. 4 is a graphical representation of the resonances characteristic of helium made with an actual working embodiment of the invention shown in FIG. 3;

FIG. 5 is a graphical representation of the resonances characteristic of helium made with an actual working embodiment of the invention shown in FIG. 3 using a system averager for enhancement of the signal to noise ratio;

FIG. 6 is a graphical representation of the 19.3 eV resonance in helium made with an actual working embodiment of the invention shown in FIG. 3 for values ofAV,= 0.03 volts and 0.016 volts;

FIG. 7 is a graphical representation of the measured square of the FWHM as a function of the square of the applied pulse height for the l9.3 eV resonance in He;

FIG. 8 is a block diagrammatic representation of the electron spectrometer shown in FIG. 3 as a laboratory gas analyzer;

FIG. 9 is a block diagrammatic representation of the electron spectrometer consisting of an electron monochromator, gas cell, and electron energy analyzer as a laboratory gas analyzer;

FIG. 10 illustrates a charged particle energy analyzer according to the invention; and

FIG. 11 is a combined block diagrammatic representation of an on line electron spectrometer gas analysis system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to the drawing and more particularly FIG. 1 thereof, there is shown a combined block-diagrammatic representation of an electron spectrometer according to the invention. An electron beam from cathode 11 passes through aperture 12 in retarder electrode 13 through input aperture 14 and exist aperture 15 of gas cell 16 for reaching Faraday collector electrode 17. A phase sensitive detector 21 is energized by the signal from Faraday detector 17 and receives a reference signal from variable height square wave pulser 22 which also applies a square wave to retarder electrode 13 to achieve RPD operation in the manner described below.

Referring to FIG. 2, there is shown in FIG. 2a the axial electron energy distribution function N (V,) centered at a potential V, as a function of energy with cutoff occurring at the potential V, applied to the retarding electrode 13. FIG. 2b shows a similar distribution function N (V, AV,) centered at V, as a function of energy with a cut applied by the potential (V, AV,) at the retarding electrode 13. FIG. 2c shows a square wave applied to the retarding electrode 13 by source 22 as a function of time with the static potential V superposed. FIG. 2d shows the resulting distribution derived by differentially combining the distributions in FIGS. 20 and 2b, N (V, AV,) -N (V,.) in the region past the retarding electrode 13 as a function of energy when detected by an a-c phase-sensitive detector using the square wave from source 22 as the signal of reference phase. Reviewing the RPD principle, charged particles from the source 11 travel to retarder electrode 13 with some axial energy distribution N (V) (number of charged particles at a particular energy V), given for example by the solid curves shown in FIGS. 2a or 2b. With retarder electrode 13 at some potential V, such that the charged particles of charge e with energies smaller than eV, are repelled back toward source 11 and those with energies greater than eV, are transmitted toward gas cell I6, the particles represented by the areas under the hatched and unhatched parts of the distribution curve would be repelled and transmitted, respectively. If the particles are negatively charged and V, is made more positive by an amount AV,, the energy necessary for transmission will decrease by eAV, as indicated in FIG. 2b. The difference current, between the two settings, V, and (V, AV,) embraces particles in the energy range 2V, to e( V, AV,). If the voltage on retarder electrode 13 is varied from V, to (V, AV,.) as a function of time as represented in FIG. 20 and phase sensitive detector 21 detects beam current after retarder electrode 13, as for example at Faraday collector 17, the distribution of charged particles that phase sensitive detector 21 detects corresponds essentially to that represented by the hatched area in FIG. 2d.

The preceding discussion is valid only if the charged particles cross the retarder electrode plane with velocities perpendicular to that plane; that is to say, entirely axial. Stated in other words, the system of FIG. I monochromatizes only axial components of velocity. Thus, the fineness of resolution of the RPD method is limited by the degree to which a parallel beam of electrons can be produced. In prior art RPD monochromators an axial magnetic field aligned the electrons which thus retained transverse components of velocity, undesirably limiting the energy width of the electron beams thus produced to greater than 0.1 eV.

The present invention produces a highly parallel beam of electrons at the plane of retarding electrode 13 with velocities perpendicular to this plane for operation in zero magnetic field established by Helmholz coil 23.

Referring to FIG. 3, there is shown a scale drawing of an RPD monochromator, gas cell, and electron collector, according to the invention. A feature of the invention is that all the elements have cylindrical symmetry. All elements were dimensioned in accordance with Spangenberg curves for tubetube and tube-aperture cylindrical electron lenses described in K. R. Spangenberg, VACUUM TUBES (McGraw Hill, New York 1948) with the following exceptions: Elements 32, 13, 34, l6, l7, and 45.

The Faraday cup 17 and its shield were designed in combination to achieve two qualities:

1. A low capacity between them since this plus leads constituted the input capacity to the phase sensitive detector which was designed to be about 8 picofarads total; and

2. The ratio of the inside surface area of the Faraday cup to the surface area of the entrance aperture of the Faraday cup was designed to be high, about 500 in order to ensure efiicient collection of low energy electrons.

The Pierce electrode was used to make electrons leave the anode aperture 310 in a parallel beam (that is, an apparent point source at infinity). Element 31 makes a point image (neglecting space charge) of the anode aperture 310 at the focus position. An aperture in element 32 was placed at the position where this point image was calculated to be formed, in order to stop electrons leaving the anode aperture at large angles with the axis. The electrons were then further decelerated to the retarding plane 17 (essentially zero) velocity.

Since calculations would be of little help, the following philosophy was followed:

I. Make the retarding element 13 as short as possible physr Cally.

2. Place a highly transmitting mesh across the central plane of the retarding element 13 to fix this central plane as an equipotential in spite of field penetration from either direction.

3. Place an aperture in element 34 an equal distance from the central plane of the retarding element 13 as the aperture in element 32.

These considerations were used to require a point image in the aperture of element 34. However, by symmetry considerations that point image requires a highly parallel beam at the central plane of the retarding element 13.

The monochromator may be divided into the electron source and injection optics to the left of retarding element 13, the retarding element 13 and the extraction optics to the right of retarding element 13. The scattering cell 16 separates Faraday cup 17 from the extraction optics.

The electron source 11 comprises an oxide coated cathode in a Pierce configuration 30 of the type described in J. R. Pierce, THE THEORY AND DESIGN OF ELECTRON BEAMS (Van Nostrand, Toronto, Canada I954). Anode 31, maintained at a potential of 21 1.7 volts, accelerated the elec trons along the axis through anode aperture 310. Decelerating electrode 32, maintained at a potential of l0.8 volts, decelerates the electrons to several eV and discharges them through decelerating electrode aperture 32a for retardation at retarding electrode 13 formed of a copper mesh 33 of LPI (lines per inch) symmetrically located in electrode 13 to which the retarding potential was applied. The mesh serves to establish an equipotential in this plane despite field penetration from neighboring electrodes. It is preferably as optically transparent as possible while being rigid enough to define a plane. Its transmissivity is typically of the order of percent. For the same reason the aperture 34a in electrode 34 is symmetrical about the plane of retarding electrode 13 with the aperture 32a in decelerating electrode 32. The potential on electrode 13 is typically 1.775 volts. Second accelerating elec trode 34 accelerates the electrons back to high energy through second accelerating electrode aperture 34a. Electrode 34 is typically l0.8 volts.

Electrodes 35, 36 and 37 coact with electrode 34 to provide a beam of small diameter and angular spread at the scattering energy incident upon input aperture 14 of scattering cell 16.

The monochromatic beam then penetrates scattering cell 16 while losing some electrons through scattering in the substance in the cell at the instantaneous energy of the incident beam. The unscattered electrons exit through exit aperture 15 and pass through entrance aperture 41 of Faraday collector shield 45 and through entrance aperture 43 of the Faraday cup 17 and are collected by Faraday cup 17.

Having described the structural arrangement of the novel monochromator, its principles of operation will be discussed. The cathode 11 produces electrons that leave anode aperture 310 in a parallel beam so that the first lens comprising anode 31 makes a point image (neglecting space charge) of the anode aperture and the first focus of the second lens 32 so that the beam leaving this second lens and impinging upon the retarding plane comprises a parallel beam of electrons. An aperture 32a is located where the point image is formed to stop electrons leaving the anode aperture 31a at large angles with the axis. The retarding region near the retarding electrode 13 is preferably as short as possible. The voltages on accelerating anode 3!, second element 32, third element 13, and fourth element 34 may be adjusted to compensate for space charge modifications of predicted beam behavior and thereby maximize the degree of parallelness of the electron paths.

The extraction optics produces a beam of small cross section and angular divergence for low energies around 20 eV and for the widest possible energy range; that is, the widest possible voltage range on the scattering cell 16. If there were only one extraction lens, the apparatus would be limited to one energy. By using four lenses 34, 35, 36 and 37 a much wider energy range is achieved by only changing the voltages on the various elements and by using the technique of raising the beam energy to a value much larger than the final energy with third accelerating electrode 36 and then decelerating it back to the scattering energy in a single step with final decelerating electrode 37. By maintaining the potential on electrode 37 substantially the same as that on scattering cell 16, there is negligible field penetration into scattering cell 16. The energy of the beam used to probe the scattering cell is thus essentially determined by the potential on electrode 37 and scattering cell 16 relative to the potential on cathode 11.

The deflectors comprise two pairs of pieces 31b and 31c for vertical deflection and 31d and 31c for horizontal deflection insulatedly separated from and cylindrically concentric with anode 3i and two pairs of elements 36b, 36c for vertical deflection and 36d, 36c for horizontal deflection insulatedly separated from and cylindrically concentric with the second accelerating electrode 36.

The horizontal deflection elements, in space quadrature with the vertical deflection elements, are not shown in the drawing so its not to obscure other structural details, these deflection elements being well known in the art and the same as the vertical elements, but rotated through a quadrant. The voltages applied across the pairs 31b, 31c and 31d, 31c and 36b, 36c and 36d, 36 are such that the potential at the axis of each pair equals the voltage applied to the corresponding lens elements 3l and 36.

Stated in algebraic terms, if the potentials on the upper and lower vertical electrodes are VI and V2, respectively; on the left and right horizontal electrodes, V3 and V4, respectively; and on the associated lens element, V,,,

Thus, an electron on axis receives no deflecting forces while an electron off axis receives a deflecting force urging it back toward the axis. The deflecting electrodes thus help establish parallel electron paths.

Typical voltages given with respect to cathode 11 V, Pierce element -370 are:

Vcathode -50 to ground F C =ground Alignment is accomplished by adjusting the lens element potentials to calculated values and then trimming these potentials until collector beam current is a maximum. Alternately the Faraday collector electrode could be coated with phosphorescent material and the potentials trimmed to produce the sharpest visible luminescent spot. After alignment of a working model of the invention, the electron beam current was found to increase 10" amperes per electron volt for AE= eA V, of the order of 0.1 eV for energies from 0 to 1 eV. For greater electron energies the current is a very slowly increasing function of electron energy.

For energies greater than about 20eV the ratio of beam current I to energy spread FWHM (Full Width at Half Maximum Current), AE was found to be almost a constant such that l/AE is approximately equal to ID" amperes per electron volt. Vacuum system 41 functions to evacuate chamber 42 which accommodates the electron gun, scattering cell l6 and Faraday collector 17. The gas inlet to scattering cell 16 is through a copper tube from the gas inlet value. The gas outlet is through the two apertures of the gas cell 16. Thus, a differential pumping ratio of about 3,000 5,000 was maintained.

The vacuum system is preferably of all nonmagnetic stainless steel construction. With an oil dififusion pump of 3,000 liters-per-second pumping speed in connection with a zeolite trap, for example, a differential pumping ratio between vacuum chamber and gas cell of more than 3000 was achieved. After 300 C. baking for l2 hours, the system reached a base pressure of about l0" Torr.

The gas pressure in the interaction region in the scattering cell was detemiined with a high pressure ion gauge corrected for the calibration constant as specified by the manufacturer. Several runs were made at various pressures to check the pressure independence of the various structures with the pressure in the interaction region of the scattering cell adjusted to attenuate the transmitted current by about a factor of 10.

The gun elements were constructed of nonmagnetic stainless steel, except electrodes 32, 33 and 34 were made of copper. Care was taken to avoid stainless steel magnetization during machining, and all parts were carefully checked for residual magnetization.

Apertures were made from 0.1 mm molybdenum sheet except for entrance and exit apertures of the scattering cell 16 which were made of 0.55 mm stainless steel. All apertures were 1 mm in diameter except for aperture 43 at the entrance of Faraday cell 17 which was 2 mm in diameter. The Faraday cup was insulated by quartz insulator 44 from its shield 45. Gun elements were spaced and aligned by clamping them to two aluminum oxide rods of ultra precision ground shafting. The two rods were held by a slotted frame to form an optical bench configuration.

Helmholtz coils compensated for the earth's magnetic field. The electron gun was operated in an a-c mode with the retarding voltage V, modulated by superimposing a square wave of amplitude AV and frequency of about 700 Hz.

Phase sensitive detector 21 detected the in-phase signal arriving at Faraday collector 17 as measured relative to a sine wave reference signal that was also used to drive variable height square wave pulser 22 that produced pulses having rise and fall times of less than nanoseconds.

The electron energy in the scattering region was varied by connecting a variable speed electronic ramp generator between cathode 11 and scattering cell 16. The transmitted current (the output of the phase-sensitive detector) was directly plotted as a function of energy on an XY recorder.

FIG. 4 illustrates such a recording for helium at a pressure of -0.3Torr. Alternately the output signal from phase-sensitive detector 21 could be fed to an averaging computer so that many sweeps could be averaged to improve the signal-to-noise ratio. A command pulse generator would then synchronize the ramp generator with the averaging computer. FIG. illustrates the output of the averaging computer for the sum of 80 sweeps of the energy range.

The performance of the invention was checked against the well-known [9.3 eV helium resonancethat produces about a l0 percent increase in transmitted energy with an electron beam energy spread of about 0. 1 eV, the true width believed to be below 0.01 eV. However, Doppler broadening caused by thermal motion of the target atoms increases the width observable with a monochromatic beam to about 0.028 eV. The 19.3 eV resonance is a good test of the degree of monochromatization for beams with full widths of 0.01 eVor greater, for any broadening of the resonance above the 0.028 eV produced by Doppler broadening would be caused by the energy width of the electron beam.

T0 line up first set all the voltages on the various electrodes at their calculated values, typically -50 volts with respect to ground for cathode l i, all the other voltages being with respect to cathode 11, 211.7 volts for anode 31, 197.2 volts for deflector 31b, 2256 volts for deflector 31c, 213.6 volts for deflector 31d, 209.4 volts for deflector 31c, 10.8 volts for decelerating electrode 32, the retarding potential of 1.775 volts on electrode 13, 10.8 volts on electrode 34, 65.2 volts on electrode 35, 261.4 volts on electrode 36, 26l.7 volts on deflector 36):, 260.9 volts on deflector 36c, 256.8 volts on deflector 36d, 265.8 volts on deflector 36c, and 25 volts on electrode 37, with AV, 0.5 volts.

The retarding potential depends on the contact potential between the cathode and the retarding electrode and was adjusted to maximize the current to Faraday collector 17. The retarding potential was adjusted to maximize collector current in each step of the lineup procedure.

Then the voltages on the deflecting electrodes 36b, 36c, 36d, 36: and electrodes 35, 36 and 37 were adjusted to max imize collector current. The electron beam width was then reduced by lowering AV from 0.5 to a value of AV equal to the desired resolution, changing the voltages on electrodes 31, 32 and 34 by a trial and error procedure to a final desired value, trimming the retarding potential and readjusting the potentials on the elements 35, 36, 36b, 360, etc. and 37 in the extraction optics.

Referring to FIG. 6, there is shown two recorder tracings of a resonance in helium taken with a square wave modulation on retarding electrode 13 of 0.03 volts and 0.016 volts, respectively. The width W, the full width at half maximum current, of either of these tracings is determined by three factors, T the natural width of the resonance, D the Doppler width and p, the pulse height; equal to AV,.

The Doppler width D for a beam gas system is given by;

D 3.338 (m/MEkT)" D= 3.338 mi/WEN) D= 3.338 (ml/(MEkTW where m is the mass of an incident particle, M is the mass of a target gas particle, E is the incident energy of the incident particle, k is the Boltzman constant and T is the absolute gas temperature. The above expression is the FWHM for a Gaussian distribution of velocities. If all three widths which determine the measured width W were characterized by Gaussian distribution, W would be given by W p P D. Although this expression is not precise for non-Gaussian distributions, it is accurate enough to determine an approximate value for r by extrapolating to zero P and knowing the value of D when P and F are smaller than D.

Referring to FIG. 7, there is shown a plot of the square of the FWHM of the 19.3 eV helium resonance as a function of P for values of P down to 0.008 eV. The extrapolation to P=0 yields a width of this resonance of 0.008 10.002 eV.

With the Doppler widths in helium at this energy of 0.028 eV, it would be difficult to detect energy resolutions of the helium resonance higher than about 0.008 eV with a beam-gas system. However, by applying the principles of the invention to a system composed of an atom beam and an electron beam in which orthogonal beams intersect in a scattering region, still higher resolution could be achieved because the Doppler broadening would then only be a function of the lack of perpendicularity of the two beams at their intersection.

The energy callibration was made from the positions of the onsets of the S and S states of helium corresponding to 19.818 and 20.614 eV, respectively, placing the transmission maximum of He'(S,,,) at l9.30:t0.0l eV. This energy scale agreed with that obtained by subtracting the contact potential as determined by the d-c bias voltage on the retarder electrode 13 from the voltage difference between the scattering cell 16 and the cathode 1 l.

The zero energy level was taken as corresponding to the voltage on retarder electrode 13 just required to establish a sharp increase in collector current. This voltage agreed within 0.05 V with the position of zero energy as determined by the voltage on retarder electrode 13 and that determined from the position of fie 8 An actual embodiment of the invention was tested over an operating energy range of 0-60 eV. It is believed that there is no upper limit to the useful energy range.

Electron spectroscopy can be used to obtain the composition of the various component constituents of a gas, solid or liquid. If one uses a monoenergetic source of electrons, there are certain advantages as compared with optical spectrometers.

1. A monoenergetic source of electrons may be easily adjusted to any desired energy with output independent of ener- 2. There is a much larger interaction between electrons and matter than between light and matter. Furthermore, interaction between electrons and matter do not have to obey as many quantum mechanical selection rules and therefore some states accessible to electrons are not accessible to light (photons). Furthermore, a device which is a source of monoenergetic electrons can be turned around and thus be made into a detector for the energy dispersion of a group of electrons. Such an energy analyzer could be used in analogy to an analyzer for electromagnetic emission spectroscopy. However, in this case of electron spectroscopy the response can be made essentially independent of the energy which would be analogous to making an optical spectrometer independent of wavelength, something that has not yet been done.

Thus electron spectroscopy can be used to look at excitation effects by impinging a beam of electrons on a sample and looking at the energy loss spectra of the scattered electrons. For each electron which excites a constituent suffers a loss in energy which corresponds to the excitation energy of that particular constituent. However, there also are resonant effects in electron scattering which can be used to obtain the signatures of the constituent atom and molecules. At certain particular electron energies a particular atom or molecule can absorb an electron, and subsequently after an extremely short time l0"a%, sec.) re-emit the electron:

a. Either in such a manner that the re-emitted electron has the same energy after re-emission as before absorption;

b. or in such a manner that the re-emitted electron has some lower energy after re-emission as before absorption the energy difference being due to the energy necessary to excite the particular final state of the atom or molecule involved.

In either case these types of resonant effects occur over extremely narrow ranges of electron energies characteristic of the particular atom or molecule involved and, hence can be used to obtain a quantitative signature of the various atoms or molecules involved.

Let us consider a beam of monoenergetic electrons impinging upon a gas composed of N constituents each of which has a certain probability of removing an electron from the beam which we express as 0,; the total scattering cross section for the r" constituent dx l the number of electrons lost from the beam in passing a thickness of gas dx, dl is given y l l S G;(E)n dx at energy E where n, is the density of the r" constituent of the gas and the current is a function of the electron energy through the cross section N l l=I..(E) exp (2 o' rmx) where we interpret [,(E) as the electron current entering the gas, and 1(E) is the electron current transmitted through the gas after a distance of x of gas. The change in current with energy due to a change in cross section with energy can be written if we can make a source of electrons with output a very slowly varying function ofenergy then we may rewrite the above as SKEW/(E) 2 n xSmlE) because the first term is almost zero and the cofactor of the summation in the second term is [(E).

This new equation says that a device can be made which measures partial pressures by observing the fractional change in transmitted current as a function of electron energy. The only requirement of such a device is a knowledge ofthe values of 6a,(E) for the various gases which is a function of the resolution of the instrument. The sensitivity of such a device depends upon the signal-to-noise ratio achievable in the mea surement of current and an upper limit of total gas density which is given by the density at which multiple scattering initiates for a particular value of x. These conditions place the sensitivity of the device to fractional partial pressures of about 10 for the state of the art value of signal to noise of 10'=( 1/61) and an upper limit of density times path length of about 2 5 l0 particles/cm. The equation further states that a small amount of one constituent can be observed in the presence of a large amount of others since the resonances characteristic of two different constituents will occur at different energies.

A system according to the invention is useful for identification and measurement of concentrations of specific constituents of gases in the manner described above. The invention may also be used for many other purposes. For example, it may be used to detect the presence of a substance having a reasonance at a predetermined energy, or it can be used for direct control of a process because the change in the transmitted electron current at resonance is a function of the concentration of the substance whose resonance is being detected. The signal detected may be used to control the amount of a particular substance in a process. A number of different schemes may be used. Charged particles from the monochromatic source may interact with the gas to be studied, and the transmitted particles collected. Such an arrangement is shown in FIG. 8. The charged particles may interact with the gas to be studied and the energy of the transmitted particles analyzed by an energy analyzer. Such an arrangement is shown in FIG. 9. Altemately the same analysis may be conducted with the scattered electrons. FIG. 10 shows a possible arrangement of the elements of the charged particle monochromator adapted for use as a charged particle energy analyzer.

FIG. ll shows a possible arrangement of the spectrometer as used for an on line process control gas analyzer system.

The invention may also be iiseful in connection with providing a high density electron beam that will produce a sharp spot on a phosphorescent screen with a low anode potential. A display tube incorporating this aspect of the invention would have a number of features. Cost and complexity of high voltage power supplies would be reduced. Less deflection power would be required to achieve a given absolute magnitude of deflection with a resultant reduction in deflection circuitry costs, power dissipation and power supply requirements.

The invention may be useful as an environmental pollution detector due to the extreme sensitivity of the device to the detection of small quantities of pollutant gases in the presence of large quantities of the normal atmospheric constituents.

The invention may be useful in body fluid analysis to determine quantities as partial pressures of gases in blood through the use of a suitable catheter and a semipermeable membrane interposed between the patient and the gas inlet valve. The device could also be used to continuously monitor the gas concentrations contained in a patients respiration cycle.

The invention may be useful in oil exploration. in this connection the device would be used to look for seepage of helium from the ground as an indication of the presence of oil. Therefore, unusually large percentages of helium in the air would be taken as an indication of the presence of an oil deposit in the vicinity.

The invention may also be useful in connection with providing a high density electron beam for generation of VHF or UHF power or for uses in a device to amplify such power. In this connection the properties of the invention with regard to high degree of spatial resolution, high degree of parallelism, and possible low electron energy would all be useful.

The invention can also be used for the detection of atoms and molecules normally in the liquid or solid state. This can be accomplished in two ways.

1. By inclusion of an arrangement for suitably heating the sample to sufficient temperature to vaporize part of it in order to allow the electrons to interact with the vapor.

2. By studying the scattering effects for electrons and/or ions directly with the sample in its original solid or liquid form. In this connection the energy of the electron or ion beam would necessarily be very much higher than that used in connection with the study of gases.

It is evident that those skilled in the art may now make numerous modifications and uses of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.

What is claimed is:

1. Electrical apparatus for providing a charged particle beam comprising,

a source of charged particles,

means for accelerating said charged particles,

means for forming said charged particles into a beam along an axis with the path of each charged particle parallel and adjacent to the paths of the others in a region outside an exit aperture defining an exit plane,

said means for forming including a plurality of axially displaced electrodes along said axis symmetrical about a central plane perpendicular to said axis embraced by one of said electrodes for constraining said particles to cross said central plane perpendicular thereto,

retarder electrode means for selectively preventing the flow of charged particles across the central plane defined by said retarder electrode means when said charged particles are energized with a potential less than a predetermined retarding potential,

and means for establishing said region substantially free from magnetic fields.

2. Electrical apparatus for providing a charged particle beam in accordance with claim 1 wherein said means for forming comprises,

injection particle optical means between said source and said retarding electrode including said means for accelerating for first accelerating and then decelerating said charged particles before they reach said central plane,

and extraction particle optical means between said retarder electrode and said exit aperture for first accelerating and then decelerating said particles before emitting them through said exit aperture. 3. Electrical apparatus for providing a charged particle beam in accordance with claim 2 wherein each of said injection particle optical means and said extraction particle optical means includes means for deflecting charged particles that are off the axis thereof toward said axis to establish the particle velocity perpendicular to said axis substantially zero.

4. Electrical apparatus for providing a charged particle beam in accordance with claim 2 wherein said retarder electrode means comprises a thin conducting element having means for establishing the central plane thereof as an equipotential plane.

5. Electrical apparatus for providing a charged particle beam in accordance with claim 3 wherein said retarder electrode means comprises a thin conducting element having means for establishing the central plane thereof as an equipotential plane.

6. Electrical apparatus for providing a charged particle beam in accordance with claim 4 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising suid means for accelerating charged particles and means for establishing the trajectory of said particles perpendicular to said central plane upon crossing the latter plane.

7. Electrical apparatus for providing a charge particle beam in accordance with claim 5 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising said means for accelerating charged particles and means for establishing the trajectory of said particles perpendicular to said central plane upon crossing the latter plane.

8. Electrical apparatus for providing a charged particle beam in accordance with claim 6 and further comprising,

scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed,

means for establishing the potential in said scattering cell means at a predetermined value relative to the potential in said exit plane,

collector means for receiving particles emitted through said output aperture to provide an output signal representative of the number of particles received by said collector means,

and means for utilizing said output signal to provide an indication of the relationship between at least one of absorption and scattering properties of said substance and the potential in said scattering cell. 9, Electrical apparatus for providing a charged particle beam in accordance with claim 7 and further comprising,

scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed,

means for establishing the potential in said scattering cell means at a predetermined value relative to the potential in said exit plane,

collector means for receiving particles emitted through said output aperture to provide an output signal representative of the number of particles received by said collector means,

and means for utilizing said output signal to provide an indication of the relationship between at least one of absorption and scattering properties of said substance and the potential in said scattering cell.

10. Electrical apparatus for providing a charged particle beam in accordance with claim 1 and further comprising,

scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed,

means for establishing the potential in said scattering cell means at a predetermined value relative to the potential in said exit plane,

collector means for receiving particles emitted through said output aperture to provide an output signal representative of the number of particles received by said collector means,

and means for utilizing said output signal to provide an indication of the relationship between at least one of absorption and scattering properties of said substance and the potential in said scattering cell.

11. Electrical apparatus for providing a charged particle beam comprising,

a source of charged particles,

means for forming said charged particles into a beam along an axis with the path of each charged particle parallel and adjacent to the paths of the others in a region outside an exit aperture,

retarder electrode means defining a central plane for selec tively preventing the How of charged particles thereacross when said charged particles are energized with a potential less than a predetermined retarding potential,

injection particle optical means between said source and said retarding electrode including means for accelerating for first accelerating and then decelerating said charged particles before they reach said central plane,

extraction particle optical means between said retarder electrode and said exit aperture for first accelerating and then decelerating said particles before emitting them through said exit aperture into a region and coacting with said injection particle optical means for constraining said particles to cross said central plane perpendicular thereto,

and means for establishing said region substantially free from magnetic fields 12. Electrical apparatus for providing a charged particle beam in accordance with claim 11 wherein each of said in jet:- tion particle optical means and said extraction particle optical means includes means for deflecting charged particles that are off the axis thereof toward said axis to establish the particle velocity perpendicular to said axis substantially zero.

13. Electrical apparatus for providing a charged particle beam in accordance with claim ll wherein said retarder electrode means comprises means for establishing said central plane as an equipotential plane.

14. Electrical apparatus for providing a charged particle beam in accordance with claim 12 wherein said retarder electrode means comprises means for establishing this central plane as an equipotential plane.

15. Electrical apparatus for providing a charged particle beam in accordance with claim 13 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising said means for accelerating charged particles.

16. Electrical apparatus for providing a charged particle beam in accordance with claim 14 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising said means for accelerating charged particles.

17. Electrical apparatus for providing a.charged particle beam in accordance with claim I l and further comprising,

scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed, means for establishing the potential in said scattering cell means at substantially the potential in said exit plane; collector means for receiving particles emitted through said It) nanu output aperture to provide an output signal representative of the number of particles received by said collector means,

means for cyclically varying the potential on said retarder electrode means between first and second potentials that are separated by much less than a volt to block the transmission across said central plane of more particles at one of said potentials than the other,

and means for processing said output signal in accordance with the cyclical variations of the potential on said retarder electrode means to provide an indication of the 

1. Electrical apparatus for providing a charged particle beam comprising, a source of charged particles, means for accelerating said charged particles, means for forming said charged particles into a beam along an axis with the path of each charged particle parallel and adjacent to the paths of the others in a region outside an exit aperture defining an exit plane, said means for forming including a plurality of axially displaced electrodes along said axis symmetrical about a central plane perpendicular to said axis embraced by one of said electrodes for constraining said particles to cross said central plane perpendicular thereto, retarder electrode means for selectively preventing the flow of charged particles across the central plane defined by said retarder electrode means when said charged particles are energized with a potential less than a predetermined retarding potential, and means for establishing said region substantially free from magnetic fields.
 2. Electrical apparatus for providing a charged particle beam in accordance with claim 1 wherein said means for forming comprises, injection particle optical means between said source and said retarding electrode including said means for accelerating for first accelerating and then decelerating said charged particles before they reach said central plane, and extraction particle optical means between said retarder electrode and said exit aperture for first accelerating and then decelerating said particles before emitting them through said exit aperture.
 3. Electrical apparatus for providing a charged particle beam in accordance with claim 2 wherein each of said inJection particle optical means and said extraction particle optical means includes means for deflecting charged particles that are off the axis thereof toward said axis to establish the particle velocity perpendicular to said axis substantially zero.
 4. Electrical apparatus for providing a charged particle beam in accordance with claim 2 wherein said retarder electrode means comprises a thin conducting element having means for establishing the central plane thereof as an equipotential plane.
 5. Electrical apparatus for providing a charged particle beam in accordance with claim 3 wherein said retarder electrode means comprises a thin conducting element having means for establishing the central plane thereof as an equipotential plane.
 6. Electrical apparatus for providing a charged particle beam in accordance with claim 4 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising said means for accelerating charged particles and means for establishing the trajectory of said particles perpendicular to said central plane upon crossing the latter plane.
 7. Electrical apparatus for providing a charge particle beam in accordance with claim 5 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising said means for accelerating charged particles and means for establishing the trajectory of said particles perpendicular to said central plane upon crossing the latter plane.
 8. Electrical apparatus for providing a charged particle beam in accordance with claim 6 and further comprising, scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed, means for establishing the potential in said scattering cell means at a predetermined value relative to the potential in said exit plane, collector means for receiving particles emitted through said output aperture to provide an output signal representative of the number of particles received by said collector means, and means for utilizing said output signal to provide an indication of the relationship between at least one of absorption and scattering properties of said substance and the potential in said scattering cell.
 9. Electrical apparatus for providing a charged particle beam in accordance with claim 7 and further comprising, scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed, means for establishing the potential in said scattering cell means at a predetermined value relative to the potential in said exit plane, collector means for receiving particles emitted through said output aperture to provide an output signal representative of the number of particles received by said collector means, and means for utilizing said output signal to provide an indication of the relationship between at least one of absorption and scattering properties of said substance and the potential in said scattering cell.
 10. Electrical apparatus for providing a charged particle beam in accordance with claim 1 and further comprising, scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed, means for establishing the potential in said scattering cell means at a predetermined value relative to the potential in said exit plane, collector means for receiving particles emitted through said output aperture to provide an output signal representative of the number of particles received by said collector means, and means for utilizing said output signal To provide an indication of the relationship between at least one of absorption and scattering properties of said substance and the potential in said scattering cell.
 11. Electrical apparatus for providing a charged particle beam comprising, a source of charged particles, means for forming said charged particles into a beam along an axis with the path of each charged particle parallel and adjacent to the paths of the others in a region outside an exit aperture, retarder electrode means defining a central plane for selectively preventing the flow of charged particles thereacross when said charged particles are energized with a potential less than a predetermined retarding potential, injection particle optical means between said source and said retarding electrode including means for accelerating for first accelerating and then decelerating said charged particles before they reach said central plane, extraction particle optical means between said retarder electrode and said exit aperture for first accelerating and then decelerating said particles before emitting them through said exit aperture into a region and coacting with said injection particle optical means for constraining said particles to cross said central plane perpendicular thereto, and means for establishing said region substantially free from magnetic fields.
 12. Electrical apparatus for providing a charged particle beam in accordance with claim 11 wherein each of said injection particle optical means and said extraction particle optical means includes means for deflecting charged particles that are off the axis thereof toward said axis to establish the particle velocity perpendicular to said axis substantially zero.
 13. Electrical apparatus for providing a charged particle beam in accordance with claim 11 wherein said retarder electrode means comprises means for establishing said central plane as an equipotential plane.
 14. Electrical apparatus for providing a charged particle beam in accordance with claim 12 wherein said retarder electrode means comprises means for establishing this central plane as an equipotential plane.
 15. Electrical apparatus for providing a charged particle beam in accordance with claim 13 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising said means for accelerating charged particles.
 16. Electrical apparatus for providing a charged particle beam in accordance with claim 14 wherein each of said injection and extraction particle optical means comprises respective apertured electrodes with the apertures thereof aligned along said axis symmetrical about said central plane and comprising said means for accelerating charged particles.
 17. Electrical apparatus for providing a charged particle beam in accordance with claim 11 and further comprising, scattering cell means having input and output apertures for receiving particles through said input aperture from said exit aperture and carrying a substance to be analyzed, means for establishing the potential in said scattering cell means at substantially the potential in said exit plane; collector means for receiving particles emitted through said output aperture to provide an output signal representative of the number of particles received by said collector means, means for cyclically varying the potential on said retarder electrode means between first and second potentials that are separated by much less than a volt to block the transmission across said central plane of more particles at one of said potentials than the other, and means for processing said output signal in accordance with the cyclical variations of the potential on said retarder electrode means to provide an indication of the relationship between at least one of absorption and scattering properties of said substance and the potential in said scatTering cell means over an energy bandwidth corresponding substantially to that between said first and second potentials.
 18. Electrical apparatus in accordance with claim 17 wherein said collector means comprises a Faraday cup electrode formed with an entrance aperture of area much less than the surface area of the cup to ensure efficient collection of low energy particles. 